Reactive adhesives, for example, isocyanate functional (polyurethane or urea forming), epoxy based and silanol condensation based adhesives, are used for bonding a wide variety of substrates together. Such adhesives are utilized in construction, vehicle manufacture, assembly of electronic subassemblies and devices, toys and the like. Such adhesives have found widespread use because they allow for reasonable processing conditions and exhibit good adhesion to many substrates. For many substrates the processes used to assemble parts using reactive adhesives require cleaning of the substrate surface to remove contaminants, sometimes the system used to clean the surface is referred to as a wipe. In many processes a primer or activator is utilized to facilitate adhesion of the adhesive to the substrate. A primer or activator system typically contains a film forming resin, a solvent and one or more of an adhesion promoter, catalyst, curing agent or crosslinking agent. See for example DE 19924139. Primers require the formation of a film on the substrate surface which bonds to the substrate surface and to the adhesive surface. In most commercial applications, primers require a minimum amount of time between application of the primer and application of the adhesive to allow adequate film build up. If an adhesive is applied before such minimum time, the primer will not have the opportunity to form a coherent film and there is a significant risk that the primer will not bond to either of the substrate surface or the adhesive surface. Many primers are moisture cure primers which require the presence of moisture to form an adequate film. Examples of such primers are U.S. Pat. No. 5,115,086; U.S. Pat. No. 5,238,993; U.S. Pat. No. 5,466,727; U.S. Pat. No. 5,468,317; and U.S. Pat. No. 5,792,811, all incorporated herein by reference. Non-film forming solutions of adhesion promoters, often called activators, are used which may contain groups intended for reaction with the substrate and the adhesive. Examples of wipe systems include U.S. Pat. No. 6,053,971, incorporated herein by reference. Examples of such adhesion promoters useful in such systems are silanes, isocyanate containing compounds, titanates and zirconates such as disclosed in U.S. Patent Publications 2005/0126683 and 2006/0124225, all incorporated herein by reference.
One-part isocyanate functional adhesives typically contain one or more isocyanate functional materials in the form of polyisocyanates or isocyanate functional prepolymers and a catalyst for the cure of the adhesive. Typically, these adhesives cure as a result of exposure to moisture and are protected from moisture until applied to a substrate. When one-part polyurethane adhesives are used with a primer, the cure rate of the primer and the adhesive must be matched to allow the adhesive to link up to the primer. For primers which cure or form a film with exposure to moisture, the adhesive and the primer compete for moisture. With the adhesive applied over the primer, it is often difficult for moisture to diffuse to the primer and the primer may not adequately form a film. If the primer does not properly cure, the bond of the adhesive system to the substrate may not be sufficient for use.
A two-part polyisocyanate based adhesive comprises, in one part, a polyisocyanate or an isocyanate functional prepolymer and in a second part a curing agent and catalyst for the reaction of the curing agent with isocyanate groups. Further, the link up of the adhesive to the surface of the substrate or the primer needs to be matched. If the cure rate of the adhesive is significantly faster than the link up of the adhesive to the substrate surface or the primer surface, then poor adhesion to the substrate will occur and the bond of the adhesive system to the substrate may not be sufficient for use. Typically, the curing agent is a compound having on average more than one active hydrogen atom, for instance an amine or hydroxyl moiety. Examples of such systems include the adhesive systems disclosed in the commonly owned patent application titled COMPOSITION USEFUL AS AN ADHESIVE FOR INSTALLING VEHICLE WINDOWS filed in the United States on Jun. 3, 2007 having a Ser. No. 11/824,984 and filed in the PCT on Jun. 5, 2007 having a serial number PCT/US07/013; U.S. Pat. No. 6,965,008; EP 1433802 and EP 1578834, all incorporated herein by reference. The adhesive starts to cure when the two parts are contacted. Two-part adhesives have the advantage that they cure much faster than one-part adhesives. The problem with utilizing two-part adhesives with primers is that the adhesives can cure faster than the primer cures and not link up to the primer. Also, the adhesive which contains isocyanate groups can tie up moisture needed to cure the primer. Thus, the time period between primer application and application of the adhesive to the substrate needs to allow for adequate film build, otherwise the system may not completely link up and form good bonds along all of the layers of the adhesive system. Further, the link up speed needs to be matched to the adhesive cure rate. If either is too fast compared to the other, then poor adhesion to the substrate will occur and the bond of the adhesive system to the substrate may not be sufficient for use.
One-part epoxy adhesive systems comprise compounds having on average more than one epoxy (glycidyl ether group), a curing agent comprising on average more than one moiety which react with epoxy moieties, such as hydroxyl or amine moieties, and a latent curing catalyst. The latent curing catalyst typically becomes active when exposed to a defined elevated temperature. One-part epoxy adhesives require heat to cure and if not properly formulated can suffer from stability problems, i.e., can cure before applied. If the catalyst causes the reaction to proceed too rapidly, the adhesive may not link up to the substrate surface of, if used, primers applied to the surface of the substrate.
A two-part epoxy system comprises, in one part, compounds having on average one or more epoxy groups and, in another part, a curing agent for the epoxy and a catalyst (often referred to as an accelerator) for the cure of the epoxy compound. Typically, the catalyst is active at ambient temperatures and accelerates reaction when the two parts are contacted. The speed of cure can be adjusted to fit the application by choice of curing agent and/or catalyst. Nevertheless, in primered systems the adhesive cure speed, the link up speed and the open time need to be carefully controlled or link up of the adhesive to the primer system can be compromised. Activator systems do not address the need to balance the link-up reaction with the curing reaction.
Another common adhesive system comprises organic based polymers having siloxy groups with hydrolyzable groups bonded thereto, i.e., groups capable of silanol condensation. These adhesives are used to bond a variety of substrates including glass, painted substrates, metals and plastics having relatively high polarity surfaces. Such adhesives can be formulated into one or two-part adhesives. The one-part adhesives are moisture curable. The two-part adhesives contain a curing agent in the second part. Examples of such adhesive systems are disclosed in Mahdi et. al., U.S. Pat. No. 6,828,403; Wu et al., U.S. Pat. No. 6,649,016 and Zhu U.S. Patent Application 2008/0017296, incorporated herein by reference.
The use of primers in adhesive systems require process steps to apply the primer, provide two additional surfaces along which an adhesive system can fail and due to the open time requirement, add process time to manufacturing processes. In modern industrial processes, there is a need to move parts and to place loads on adhesive systems as soon as possible. Moving parts or articles bonded together with adhesives or placing a load on the adhesive bond before the system builds adequate strength can result in failure of the adhesive system thus resulting in faulty parts or articles. What is needed is a system for bonding parts using reactive adhesives which avoids these problems and which matches the cure rate of the reactive adhesive to the link up speed to the substrate or the primer system.